This invention relates to precipitation detectors and particularly to apparatus for detecting wet and icy conditions on the surface of a pathway, such as highway or airport runway.
Such apparatus which detects wet and icy surface conditions is of tremendous aid to motorists, highway departments, airport managers, pilots and the like. A motorist can be made aware of an icy condition on a bridge deck before crossing it and take the necessary precautions. Likewise, once an airport manager is aware of an icy condition on one or more of his runways, he can divert planes to a non-icy runway or initiate deicing procedures on the affected runways. Motorists can be warned of a light accumulation of water on a dirty highway which is a hazard almost as well known as it is difficult to see.
A number of systems have been suggested for detecting precipitation on a pathway, such as those disclosed in coassigned U.S. Pat. Nos. 3,873,927 and 3,882,381, in U.S. Pat. Nos. 2,419,266, 3,243,793, and 3,428,890, and in French Pat. No. 2,078,982.
While prior art systems in general provide useful information on surface conditions it would be highly advantageous if their effectiveness could be improved in certain respects. Where the sensor is positioned in the most desirable location to accurately sense the actual surface conditions, viz., embedded in the pathway with its top surface generally coplanar with that of the pathway, it is exposed to the traffic such as passing cars and trucks and landing aircraft, etc. A sensor unit which has capacitor plates disposed a short distance, e.g., 1/16 inch (0.16 cm.) below the top surface of an epoxy encapsulated sensor is therefore subjected to considerable wear and abrasion. Thus the relatively thin layer of epoxy tends to be worn away subjecting the capacitor plates to damage and rendering the sensor unit inoperative.
If, however, the wear resistance and useful life of the sensor is improved by increasing the thickness of the overlying synthetic resin used for encapsulation, the sensitivity and reliability of the unit to surface precipitation is markedly diminished. Moreover, the response of prior art systems is frequently subject to drift and variation as a function of temperature. These are not easily compensable and result in possible inaccuracy and imprecision in indicating the actual surface conditions.
Where the system uses a signal generator in conjunction with a sensor, it is desirable that this generator and its associated components be located at a remote position, such as under the bridge or a considerable distance from the runway or highway surface where the sensor is embedded. However, to improve the sensitivity of such systems, higher frequency (e.g., 200 KHz) generators have frequently been used and this severely limits the distance the signal can be conducted by the usual wiring cable.